Mental retardation (MR) is a condition that affects about 6 million American and over half a million Canadian children under the age of 14 years (Shea, 2006). MR is a general term for a heterogeneous group of disorders that are defined by deficits in cognitive and adaptive development. Frequently other terms used include “general learning disorder”, “mental handicap”, “learning disability”, “intellectual handicap”, and “intellectual disability” (Leonard & Wen, 2002), also “mentally challenged” and “developmental delay” (Shea, 2006). The prevalence of MR is commonly given as about 1% of the population (Szymanski & King, 1999), with a higher proportion of males to females affected (1.4:1; Murphy et al., 1998).
MR is commonly classified according to Intelligent Quotient (IQ). The Diagnostic & Statistical Manual of Mental Disorders (4th Edition; DSM-IV, 1994) identifies mild MR in the IQ range 50-55 to 70, moderate MR as 35-40 to 55-55, severe MR as 20-25 to 35-40, and profound MR as below 20-25. Attempts to understand the etiological basis of cases of MR are important because they may assist with the diagnosis of associated co-morbidities (eg. aortic stenosis in Williams syndrome), or may help with prenatal diagnostics and/or genetic counselling (eg. in fragile X syndrome), or may identify a treatable condition such as phenylketonuria (Shea, 2006). In addition, understanding the cause of MR may help families cope with a MR child and may help them access support infrastructure.
The contribution of genetics to MR has long been established. Conventionally, genetic forms of MR are subdivided into two major categories. Firstly, syndromic MR is characterized by cognitive deficits associated with other clinical and biological features. Secondly, non-syndromic form of MR in which cognitive impairment is the only manifestation of disease. Genetic factors are involved in the etiology of approximately two-thirds of mental retardation cases (Curry, 2002). In inherited forms of mental retardation, X-linkage or autosomal recessive inheritance patterns are the most plausible, since procreation from affected individuals is not common. To-date, more than 60 genes have been reported for X linked mental retardation (Chelly et al, 2006). But the molecular basis of autosomal recessive mental retardations are still poorly understood. Although autosomal recessive inheritance is estimated to be involved in nearly a quarter of all individuals with non-syndromic mental retardation (NSMR) (reviewed in Basel-Vanagaite et al,7), only four autosomal genes, the PRSS12 gene on chromosome 4q26 (neurotrypsin [MIM #606709]), the CC2D1A gene on chromosome 19p13.12 [MIM #610055], the CRBN gene on chromosome 3p26 (cereblon [MIM #609262]), and very recently GRIK2 on 6q16.1-q21 (ionotropic glutamate receptor 6 [MIM #138244]) have been reported so far to cause autosomal recessive NSMR.7-10 However, only a very few families or unrelated individuals with ARMR have been confirmed for each of these genes (PRSS12, N=1; CRBN, N=1; CC2D1A, N=9; GRIK2, N=1). The most recent of these genes, GRIK2, was discovered at one of 8 novel loci for autosomal recessive non-syndromic mental retardation (NSMR) recently mapped by homozygosity mapping in 78 consanguineous Iranian families. However, no disease gene or mutation has yet been reported for the other 7 loci (Najmabadi et al, 2007). Neurotrypsin (PRSS12) was the first gene reported in etiology of autosomal recessive non-syndromic mental retardation. The disease locus was mapped on chromosome 4q21-q25 by homozygosity mapping using a set 400 microsatellite markers across the genome. This interval encompasses about 29 genes of known function including the DKK2, PL34, CASP6, ANK2, CAMK2D TRPC3, and PRSS12 genes. A homozygous 4 by deletion in exon 7 of the PRSS12 gene was found cosegregating in all affected individuals, and resulted in a premature stop codon, 147 bp downstream of the deletion (Molinari et al, 2002). In another family with mild autosomal recessive non-syndromic mental retardation, a nonsense mutation causing a premature stop codon was identified in the CRBN gene that encodes for an ATP-dependent Lon protease. This C to T substitution changed an arginine residue to a stop codon in exon 11 (R419X) of this gene (Higgins et al, 2004). Mutations in the PRSS12 and CRBN genes have each been reported in only one family to-date. Recently a protein truncating mutation was identified in the CC2D1A gene in nine consanguineous families with severe autosomal recessive NSMR. The CC2D1A protein is involved in the calcium dependent phospholipid binding (Basel-Vanagaite et al, 2006).
A recent study has mapped 8 novel loci for autosomal recessive non-syndromic mental retardation (NSMR) by homozygosity mapping in 78 consanguineous Iranian families. However, no disease gene or mutation has yet been reported (Najmabadi et al, 2007). Another recently published study has mapped a new locus for autosomal recessive non-syndromic mental retardation to 1p21.1-p13.3 (Uyguner, 2007).
There is a need in the art to identify genetic markers associated with mental retardation. Further there is a need in the art to identify nucleotide sequences associated with mental retardation. There is also a need in the art for new diagnostic assays for mental retardation.